Mixer generate a number higher then MaxValueValidator

hi @klen I am having an error with my model when I use as a constraint Min/MaxValueValidator.

```
from django.core.validators import MinValueValidator, MaxValueValidator

class Course(models.Model):
number_of_minutes = models.PositiveIntegerField(
validators=[
MinValueValidator(12),
MaxValueValidator(60)
]
)
```
in the shell

```
course = mixer.blend(Course)
course.number_of_minutes
=> 5645
```

So I want to know how to have the MaxValueValidator constraint

Start by reproducing the issue in the Django shell with the shown Course model and Mixer call. Trace how Mixer generates values for Django model fields with MinValueValidator and MaxValueValidator; done means generated number_of_minutes values stay between 12 and 60.

Written by the indexing model from the issue text.